Decompression devices had been used in small engines before 1978. The objective of the decompression device of an engine is to control the opening and closing periods of intake and exhaust valves as the engine is operated at different engine speed. With the decompression device, the pressure of the engine's combustion chamber is partially released to make manually starting the engine up easily achievable.
Please refer to FIG. 1, which is the exploded perspective view of an automatic decompression device for four-stroke engines of the prior art, which the automatic decompression device 10 comprises a cam gear 11, a counterweight 12, a rotation device 13, a plate cover 14, and a spring 15. A cam surface 111 on the upper side of the cam gear 11 has a recess 111. The counterweight 12 has a guide slot 121 at its one side, and, at the other side, the counterweight 12 and the spring 15 are connected. The rotation device 13 includes a half cylinder 131 on its upper right side and a pin 132 on its lower left side. The half cylinder 131 has a circular surface 133 and a groove 134. The recess 112 contains the half cylinder 131 of the rotation device 13. The pin 132 is projected perpendicularly outwardly from the bottom side of the cam gear 11 and guided by the guide slot 121 of the counterweight 12. The counterweight 12, the rotation device 13, and the spring 15 are pressed between the cam gear 11 and the plate cover 14.
Before the engine is started up, since the spring forces induced by the spring 15 act on the counterweight 12, the circular surface 133 of the half cylinder 13 is projected radially outwardly from the cam surface 111 through the recess 112. As the engine is started up and its speed grows up, the centrifugal forces generated by rotation act on the counterweight 12 against the spring forces induced by the spring 15 to move the counterweight 12 radially outwardly, and the rotation device 13 is thus rotated by the counterweight 12 because the pin 132 of the rotation device 13 is guided by the guide slot 121 of the counterweight 12. At this time, the groove 134 of the half cylinder 131 is rotated to face to the outside of the cam gear 11, and the circular surface 133 is consequently rotated to face to the inside of the cam gear 11. In other words, the half cylinder 131 is not projected outwardly from the cam surface 111 through the recess 112.
Please refer to FIG. 2, that is the schematic drawing to operate the automatic decompression device shown in FIG. 1. Two lower arms 21 contact the cam surface 111 of the decompression device 10. There is a pushrod 22 connected with each lower arm 21. The upper end of each pushrod 22 is connected to an upper arm 23 which is hinged to the engine with a seat 25. In addition, an intake valve 20 and an exhaust valve 24 are fixed to the ends of the upper arms 23 away from pushrods 22. When the engine is working, the cam gear 111 is rotated and, consequently, the lower arms 21 are cyclically moved left and right since the lower arms 21 contact the cam surface 111. The motions of the lower arms 21 repeatedly drive the pushrods 22 to push and pull the upper arms 23, and then the intake valve 20 and the exhaust valve 24 are repeatedly opened and closed. When the engine just begins to be started up, the cam gear 11 is rotated slowly. At this time, since the centrifugal forces induced by rotation are too small to act on the counterweight 12 against the spring forces generated by the spring 15, the half cylinder 13 is projected radially outwardly from the cam surface 111 through the recess 112. This biases the opening and closing duty cycles of both valves 20 and 24, and then the function of the automatic decompression device is enabled, i.e., the pressure of the engine's combustion chamber is thus partially released to make manually starting the engine up easily achievable. As the engine's speed is relatively high, the centrifugal forces induced by rotation are large enough to act on the counterweight 12 against the spring forces generated by the spring 15. Consequently, the counterweight 12 is moved radially outwardly, and the rotation device 13 is thus rotated by the counterweight 12 because the pin 132 of the rotation device 13 is guided by the guide slot 121 of the counterweight 12. Now, the circular surface 133 of the half cylinder 13 is rotated to face to the inside of the cam gear 11. This makes the opening and closing duty cycles of both valves 20 and 24 normal. In other words, the function of the automatic decompression device is not enabled.
A major problem of the automatic decompression device of the prior art is that the rotation device, the spring, and the counterweight must be assembled and pressed to the cam gear by the plate cover to guarantee its normal operation. Another problem is that the operation and structure of the rotation device of the prior art are complex.
The above problems induce high cost, more complicated design, fabrication, and assembly of the automatic decompression device of the prior art. The present inventor recognized the need for providing an automatic decompression device whose design, fabrication, and assembly can be simplified, and its costs can be down.